Conventionally, there is known a technology of promoting combustion utilizing active species such as OH radical. For example, Japanese Unexamined Patent Application, Publication No. 2007-113570 discloses an ignition device employing a technology of this kind. The ignition device disclosed by Japanese Unexamined Patent Application, Publication No. 2007-113570 is provided in an internal combustion engine such as a gasoline engine. In this ignition device, fuel air mixture that has been ignited is irradiated with a microwave so as to cause plasma discharge, increase radical concentration, and promote flame propagation.